1. Field of the Invention
The present invention relates to a scintillator comprising CsBrxI(1-x) doped with europium (CsBrxI(1-x):Eu). The scintillator shows a high conversion efficiency for high energy radiation detection and a low afterglow. Therefore, the invention also relates to digital radiography flat panel detectors (FPDs) and high energy radiation detectors comprising the CsBrxI(1-x):Eu scintillator with high conversion efficiency. The present invention is useful in the X-ray imaging field, in particular where a high quality image is important and for high energy radiation detection applications as well.
2. Description of the Related Art
Inorganic scintillators are employed in most of the current medical diagnostic imaging modalities using X-rays or gamma rays.
In digital radiography (DR) flat panel detectors (FPDs), which are X-ray detectors that capture images from objects during inspection procedures or from body parts of patients to be examined, scintillators are used to convert X-rays into light. This light interacts with an amorphous silicon (a-Si) semiconductor sensor layer, where electrical charges are created. The charges are collected by thin film transistors (TFT's) which are arranged in an array. The transistors are switched-on row by row and column by column to read out and the collected charges are transformed into a voltage, which is transformed into a digital number and stored in a computer to make up a digital image corresponding with the shadow image of the irradiated object. This way of conversion of X-rays into electrical charges is called indirect conversion direct radiography (ICDR). Typical scintillating material for use in ICDR is caesium iodide doped with thallium (CsI:Tl).
Scintillators are also particularly useful for the detection of high energy radiation in combination with a photomultiplier tube (PMT). When high energy radiation interacts with the scintillator material, photons are created that can activate the PMT. The scintillation light is emitted isotropically; so the scintillator is typically surrounded with reflective material to minimize the loss of light and then is optically coupled to the photocathode of the PMT. Scintillation photons incident on the photocathode liberate electrons through the photoelectric effect, and these photoelectrons are then accelerated by a strong electric field in the PMT. The output signal produced is proportional to the energy of the gamma ray in the scintillator. High energy radiation detectors based on a scintillator and a PMT are useful in detection of radiation in gamma ray cameras and positron scanners (Positron Emission Tomography and single-photon emission computed tomography). High energy radiation detectors are also used in scintillation counting mode for measuring radiation in activation analysis, X-ray fluorescent analysis, Transmission Electron Microscopy (TEM), Time of Flight Mass Spectrometry (TOF MS), high energy physics collision detection and detection of cosmic rays. Scintillators which are coupled with a photomultiplier tube (PMT) are used in survey meters to detect radioactive contamination, monitoring and testing nuclear material.
Scintillators are also used in security, baggage cargo and personal screening. Another important application of scintillators is in dosimetry for personal safety dosimeters. Dosimeters are used to measure the radiation dose received by body, tissue and matter, received from indirect or direct ionizing radiation. It is very important that the dosimeter or other detection radiation device has a high sensitivity and can detect any level of radiation.
The scintillation conversion is a relatively complicated process that can be generally divided in three sub-processes: conversion, transport and luminescence. These three steps determine the emission efficiency of the scintillator material. In this respect, structured scintillators made of inorganic materials crystals doped with an activator element, such as sodium iodide doped with thallium (NaI:Tl) or caesium iodide doped with thallium (CsI:Tl), were developed to allow detecting and monitoring higher energy X- or γ-rays (high energy: below ˜1 MeV) and are employed in the (X- or γ-rays) photon counting regime.
The mechanism of luminescence of scintillators consists in accumulating the generated light arriving soon after the initial conversion stage is accomplished and the most important parameters determining the conversion efficiency are: (1) the light yield or conversion efficiency; (2) the X-ray stopping power; (3) the decay time; (4) the spectral match between the scintillator emission spectrum and the sensitivity spectrum of the photosensitive detector; (5) the chemical stability and radiation resistance of the scintillator; and (6) the energy resolution. The conversion efficiency of a scintillator can be measured in a relative way, i.e. by measuring the light emission of the scintillator under study and comparing the results with the measurements of a known standard scintillator, as a reference. By improving the conversion efficiency of scintillators better image quality and shorter image acquisition time can be obtained.
Despite the acknowledged advantages of CsI:Tl in many scintillator applications with respect to conversion efficiency, a characteristic property that undermines its use in high-speed radiographic and radionuclide imaging is the presence of a strong afterglow component in its scintillation decay. This causes pulse pile up in high count-rate applications, reduced energy resolution in radionuclide imaging, and reconstruction artefacts in computed tomography applications. Another disadvantage of CsI:Tl is the presence of very toxic Tl. The very toxic Tl represents an important safety issue in production of CsI:Tl based scintillators. With regard to the high energy radiation detectors based on a combination of a scintillator with a PMT, the spectrum of the emitted light of CsI(Tl) with its maximum at 550 nm does not match very well the sensitivity spectrum of the photocathode of the PMT having a maximum between 400 and 450 nm.
Europium doped caesium bromo iodide (CsBrxI(1-x):Eu) based scintillators, with a high content of iodide (x<0.5) show a very low afterglow level, do not include a highly toxic Tl activator and their emission spectrum matches well the sensitivity spectrum of the photocathode of a PMT. CsBrxI(1-x):Eu based scintillators however do not show a high conversion efficiency. It is thus desirable to increase the conversion efficiency of CsBrxI(1-x):Eu in an easy and reliable manner. Therefore, research has been done to improve the conversion efficiency while maintaining the advantage of a low afterglow level of europium doped CsI material.
U.S. Pat. No. 7,560,046 relates to a scintillator material that increases the conversion efficiency by avoiding the production of radiation damages that can lead to the occurrence of ghost images. Therefore, this document discloses an annealed scintillator composition with a formula of A3B2C3O12, where A is at least one member of the group consisting of Tb, Ce, and Lu, or combinations thereof; B is an octahedral site (Al), and C is a tetrahedral site (also Al). One or more substitutions are included. These materials do not include alkali halide compounds doped with at least one activator compound.
Cherginets et al. (Luminescent properties of CsI single crystals grown from the melt treated with EuI2—V. L. Cherginets, T. P. Rebrova, Yu. N. Datsko, V. F. Goncharenko, N. N. Kosinov, R. P. Yavetsky, and V. Yu. Pedash Cryst. Res. Technol. 47, No. 6, 684-688. 2012) studied the scavenger properties of super-pure alkaline earth halides, namely CsI single crystals compositions doped with different concentrations of europium in the form of EuI2, from 10−4 to 10−2 mol·kg−1. The luminescent properties of CsI:Eu crystals are attributed to the distortion of the crystal lattice and not necessarily to the doping of Eu. The proven improvement of the Eu dopant is the reduction of afterglow by suppression of the slow components of the scintillator pulse, which occurs at EuI2 concentration in CsI melt equal to 0.01 mol·kg−2.
Thacker et al. (Low-Afterglow CsI:Tl microcolumnar films for small animal high-speed microCT—S. C. Thacker, B. Singh, V. Gaysinskiy, E. E. Ovechkina, S. R. Miller, C. Brecher, and V. V. Nagarkar, Nucl. Instrum. Methods Phys. Res. A. 2009 Jun. 1, 604(1), 89-92) discovered that adding a second dopant Eu2+ to CsI:Tl reduces afterglow with a factor of 40 at 2 ms after a short excitation pulse of 20 ns, and with a factor of 15 at 2 ms after a long excitation pulse of 100 ms. The Eu is added to reduce the afterglow, and it is not used as a scintillator activator.
EP1113290 relates to the improvement in output decrease over time of the scintillator, by adding a light absorbing layer between the scintillator and a light sensitive imaging array. This is to reduce the rate at which the light sensitive imaging array saturates, to reduce light incident on the switching devices, and/or to compensate for the aging of a scintillator. The invention is related to the improvement of the efficiency after degradation due to operation.
U.S. Pat. No. 7,558,412 discloses a method for detecting the potential of an X-ray imaging system to create images with scintillator hysteresis artefacts. The method comprises measuring signal levels for different areas of interest and comparing all measurements with a given threshold to determine if scintillator hysteresis artefacts may be produced by a certain scintillator in result from a large x-ray flux dose. Said effect can occur even in scintillators including CsI doped with TI (CsI:Tl). U.S. Pat. No. 7,558,412 further discloses that the method may optionally include exposing the scintillator to an x-ray flux if the difference between the two signals obtained is greater than a given threshold and thus detecting the potential of said scintillator to produce image artefacts. The method is connected to “bleaching” of the scintillator to the original level of efficiency (before irradiation) and not adding it.
In Gektin et al. (Radiation damage of CsI:Eu crystals. Functional Materials, 20; n.2 (2013)—STC “Institute for Single Crystals” of National Academy of Sciences of Ukraine) a study is presented on the radiation damage and afterglow nature for CsI:Eu crystals concluding that the luminescence parameters depend on the X-ray irradiation conditions and that irradiation leads to emission suppression at doses less than 100 Gy when the induced absorption was not observed yet.
In another document, Gektin et al. (Europium emission centers in CsI:Eu crystal. Optical Materials 35 (2013), 2613-2617) the absorption, excitation and luminescence spectra of pure and Eu doped single crystals were studied depending on the activator content, the excitation energy, the heat treatment and the X-ray radiation. It is shown that the structure and concentration of the complex centres changes at heat treatment. Only an increase of annealing temperature from 300° C. to 405° C., followed by quenching, has a marked influence on the spectral composition and intensity of luminescence.
Giokaris et al. (Nuclear Instruments and Methods in Physics Research A 569 (2006) 185-187) compared scintillators based on CsI:Tl and CsI:Na crystals, coupled with Position Sensitive Photomultiplier Tubes (PSPMTs) for gamma-ray detection with respect to their performance in terms of sensitivity, spatial and energy resolution. CsI:Na based scintillators are very hygroscopic and hence difficult to coat via a dispersion. CsI:Tl based scintillators have a light emission spectrum which matches less good with the sensitivity spectrum of the PMT than CsI:Eu and CsI:Tl crystals are obtained after a long and hence expensive production process.
Document EP1944350A2 discloses the method of optimizing speed of storage phosphor needle image plates (NIP), particularly europium doped caesium bromide (CsBr:Eu), by annealing. The object of this patent is realised with the marker of stable Eu-ligand complexes measured with electron paramagnetic resonance (EPR). The peaks of the EPR signal are measured at the frequency of 34 GHz and the following flux density of magnetic filed are specified: 880, 1220, 1380 and 1420 mT. Europium doped caesium bromide (CsBr:Eu) is very hygroscopic and the methods of applying this material onto a substrate are therefore not compatible with a coating process from a coating dispersion.
EP2067841 discloses a phosphor storage plate based on a binderless needle-shaped Cs(X,X′), X representing Br and X′ representing F, Cl, Br, I but no specific combination of X being Br with X′ being I is disclosed.
However, none of these documents discloses a method of production that increases the conversion efficiency of CsBrxI(1-x):Eu as a scintillator and maintaining the advantage of a low afterglow level of europium doped CsBrxI(1-x) material.